Technical Abstract:
The variable source area (VSA) concept provides the underlying paradigm for managing nutrient losses in runoff in the northeastern U.S. This study sought to explain factors controlling runoff generation and losses of nitrogen and phosphorus along a single hillslope with contrasting soils and nutrient applications. The experimental hillslope is located within a 27.4 ha agricultural watershed in the Ridge and Valley Province of central Pennsylvania. A total of 94 storm events were monitored from 2002 to 2004 using runoff monitoring plots (2 m x 1 m) established in three distinct landscape positions. The seepage-slope and transportational mid-slope plots were located in well-drained residual soils, whereas the foot-slope plots were located nearest the stream channel in somewhat poorly drained colluvial soils with fragipans. During the study period, manure nutrients only were applied to fields within and upslope of the seepage-slope position. Results of the study showed that storms occurring in residual soils generated small volumes of infiltration-excess surface runoff that resulted in high nutrient concentrations but relatively small nutrient losses. In contrast, the foot-slope position produced much larger volumes of primarily saturation-excess surface runoff that significantly diluted nutrient concentrations, but led to much greater nutrient losses. The overall trends in this study demonstrate the importance of hydrology as an important controlling variable on nutrient losses in agricultural watersheds.